Hospitals could end up overcrowded on Thursday as they struggle to discharge patients because of the junior doctors strike, NHS leaders are warning.
The NHS in England seemed to cope well on Wednesday following the walkout from 08:00 GMT over the contract dispute.
But NHS England said the second day of the 48-hour walkout was always going to be more difficult.
Officials said hospitals might struggle to discharge patients without junior medics on wards.
More than 5,000 operations were postponed ahead of the action to try to relieve pressures.
But Dr Anne Rainsberry, who is in charge of planning during the strike, suggested hospitals might find it difficult to discharge patients, which could then create a backlog in hospital wards.
She said this was because they had a "valuable role" in chasing up test results and ensuring patients were ready for discharge.
"So far the NHS is holding up, but we always expected the second half of the strike will be more challenging," she said.
She also urged patients to go to hospital only when absolutely necessary.
"If people need medical help and it's not an emergency they should consider NHS Choices, visit their local pharmacy, or call their GP or NHS 111 for more serious matters.
"If their condition is an emergency or life-threatening they should call 999 as usual or go to A&E."
Doctors are providing emergency cover during the walkout, which ends on Friday morning, and consultants, nurses and midwives are all working in hospital as normal. GP surgeries are largely unaffected.
Dr Johann Malawana, the BMA's junior doctor leader, said the strike had been well supported by the profession with 147 picket lines taking place.
"We deeply regret the disruption to patients and we remain open to talks with the government to try and resolve this dispute."
But he added: "If the government wants more from an already overstretched workforce it will need to ensure there are more doctors, nurses and healthcare professionals, alongside additional funding. Ministers have failed to provide any detailed plan as to how their vague and unclear plans will be delivered."
The government has said there is no extra money for junior doctors.
On both Wednesday and Thursday, just over half of junior doctors expected in work did not turn up - a figure broadly in line with the previous strikes and to be expected considering the numbers needed to provide emergency care.
How far apart were the two sides?
- The BMA wanted everyone who worked on a Saturday to be paid at 50% above the basic rate
- Ministers only offered extra pay after 17:00 and at a lower rate of 30%
- But they have agreed to top up the pay by 30% for those who work regular Saturdays - defined as at least one in four
- Agreement was also not reached on on-call allowances, how limits on working hours are to be policed and days off between night shifts
- The government offered a basic pay rise of 13.5%
- The BMA has said it was willing to accept between a 4% and 7% rise in basic pay to cover more generous weekend pay
The latest walkout is the third in the long-running dispute, but the first to last 48 hours.
So far 19,000 operations and treatments have had to be postponed because of industrial action. The NHS carries out about 30,000 procedures a day.
Thousands of check-ups, appointments and tests have been affected as well.
This week's walkout is the first of three 48-hour stoppages planned by the British Medical Association as it continues its fight against the government's plans. The next two are planned for April.
The union has also said it will be launching a legal challenge to oppose the imposition of the contract that was announced following last month's strike.
But ministers have said they will be pushing ahead with imposition regardless. The new contracts are due to go out in May and will come into force from August.
Danny Mortimer, chief executive of NHS Employers, which represented the government in talks, said it was "hugely disappointing" that doctors were taking action as the contract was "safe, fair and reasonable".
He said the strike would be causing "much disruption to patients and their families which is completely unnecessary".